What is the term for the lowest elevation in a stream bed?

The term that refers to the lowest elevation in a stream bed is thalweg. The thalweg is a significant hydrological feature, as it represents the line connecting the lowest points along the entire length of a stream or river. This line typically runs through the deepest part of the channel and is crucial in determining water flow and the behavior of the stream during various hydrological conditions.

Understanding the thalweg is important in the context of bridge safety inspections, as changes in the thalweg can indicate erosion or sediment deposition, which can affect the stability of the structures above. Recognizing this term and its implications helps inspectors assess potential risks related to the bridge's foundations and the overall health of the waterway.

The other choices relate to various features of waterways or flood management but do not specifically denote the lowest elevation in a stream bed. Streambank refers to the land alongside the stream, flood plain indicates areas that are subject to flooding adjacent to the stream, and freeboard is a term used to describe the height of a structure, like a bridge, above the water level.